1. Field of the Invention
The present invention relates to an improved method and apparatus for preparing a substrate sheet for optical recording media such as optical discs and optical cards by continuously transferring a preformat pattern onto the surface of a resin sheet.
2. Related Background Art
In order to prepare substrates for optical recording media such as optical discs and optical cards which have a concave and/or a convex information structure (hereinafter referred to as "preformat") on their surfaces, a method using an extrusion molding technique has been investigated in recent years. That is, this method comprises pressing a roll stamper having a pattern (hereinafter referred to as "preformat pattern") corresponding to the preformat on its peripheral surface against the surface of a molten resin sheet extruded from an extruder to continuously transfer the preformat pattern to the resin surface, whereby the mass productivity of the substrates for the optical recording media can be remarkably improved. Therefore, much attention is paid to this method.
However, in the case that the substrates for the optical recording media are prepared, it must solve two technical problems, i.e., the improvement of a transfer precision of the preformat pattern and the reduction of a birefringence of the substrates irrespective of a molding method.
In the method of molding the substrates for the optical recording media by the use of the extrusion molding, it is known that molding conditions such as a roll temperature, a gap between the roll stamper and the surface of a molding roll, and an extrusion rate of the resin from a die have a large influence on the transfer precision of the preformat pattern and the birefringence of the substrates. For the purpose of solving this problem, the present inventors have suggested, in EP 478372, the employment of a roll stamper in which a heat insulating layer is interposed between the peripheral surface of a roll base and a stamper so as to obtain the high-quality substrates even under the fluctuation of molding conditions. This suggested method is extremely effective, since it can provide the high-quality substrates even when the molding conditions slightly fluctuate. However, in order to stably achieve the further improvement of the transfer precision of the preformat pattern to the substrates and the further reduction of the birefringence of the substrates, it is necessary to positively control the molding conditions.
However, the control of the molding conditions for stably achieving the improvement of the transfer precision of the preformat pattern to the substrates and the reduction of the birefringence of the substrates has depended largely upon the experience of an operator so far, and therefore it is difficult to stably manufacture the substrates for the optical recording media having a higher quality.